Natural gas (of which the primary component is CH4) contains lesser amounts of higher hydrocarbons such as alkanes and alkenes (or the general class of C2-C6+ hydrocarbons) which are prone, during catalytic processing such as pre-reforming and reforming reactions, to form coke deposits and deactivate the catalyst.
Coke formation often accompanies high temperature conversion processes that utilize hydrocarbon feed streams, and is detrimental to the operational efficiency of hydrocarbon reforming equipment. For example, the available reactive surface area of the reforming catalysts can be decreased by the undesirable deposition of coke on the surface of the catalyst. The deposition of coke on process equipment can also lead to inefficiencies in heat transfer, as well as unwanted pressure drops.
Difficulties associated with coke formation are of particular concern in reformers used for providing hydrogen to fuel cells since applications such as fueling stations and residential applications often mandate smaller scale reformer designs and a minimization of maintenance requirements. As such, equipment and maintenance provisions for the removal of coke that are available in an industrial setting such as in an ammonia plant are effectively unavailable for many fuel cell reformer applications.
The reforming or pre-reforming of ethane, as a surrogate for higher hydrocarbons is shown in the equations below.Reforming: C2H6+2H2O5H2+2COPre-Reforming: C2H6+2H2O3H2+CO(CO2)+CH4 Reforming is practiced in chemical plants designed to maximize the production of H2 and CO from all hydrocarbons present in the feed while pre-reforming is mainly practiced at lower temperatures than reforming primarily to remove higher hydrocarbon coke precursors forming CO, H2, and CH4. Both pre-reforming and reforming can be practiced at a variety of pressures. Reduced nickel catalysts (such as Ni/Al2O3) are commonly used for reforming reactions. However, nickel catalysts are highly susceptible to deactivation by small amounts of sulfur present in the feed. Deactivation is caused by nickel sulfide (NiS) formation which poisons the active Ni metal sites over time. The active Ni metal sites cannot be conveniently regenerated, and thus the deactivation process is essentially irreversible. Consequently, it is common practice to desulfurize the hydrocarbon feed prior to reforming. The hydrocarbon feed is desulfurized by catalytic hydrodesulfurization using Co, Mo/Al2O3 catalysts at temperatures in excess of 350° C. and pressures above 300 psig. One concern with such a catalytic hydrodesulfurization is the production hydrogen sulfide (H2S) which is then adsorbed on ZnO downstream in the following manner.
The necessity for sulfur removal is a critical limitation with the reforming process to avoid poisoning of downstream catalysts and equipment and thus large volumes of ZnO or other suitable adsorbents must be present in the process stream upstream from the reformer. These adsorbents have limited capacities for adsorbing hydrogen sulfide, and thus the adsorbents must be replaced frequently. The capacity of an adsorbent for adsorbing hydrogen sulfide is decreased with H2O in the feed gas, as well as temperature. The presence of an adsorbent in the process stream adds significantly to the overall pressure drop and process complications. This process is quite complicated and requires costly regeneration or disposal of the catalyzed-reactive hydrodesulfurization bed and replacement of sulfur saturated ZnO.
Furthermore, sulfur removal is an important aspect in petroleum refining processes such as catalytic reforming, which play an integral role in upgrading straight run or cracked naphtha feedstocks, as by increasing the octane number of the gasoline fraction contained in such feedstocks. To achieve maximum run lengths and increase process efficiency, it is generally recognized that the sulfur content of the feedstock must be minimized. Reforming catalysts, and particularly those comprising platinum, and most particularly comprising platinum and rhenium, deactivate rapidly in the presence of sulfur compounds, and as a result, it is necessary to reduce the sulfur content of reformer feedstocks as low as possible.